internal/trace: move maxArgs into tracev2 and validate specs

internal/trace: move maxArgs into tracev2 and validate specs

This change moves maxArgs to tracev2 and renames it MaxTimedEventArgs.
It also updates the tests to make sure the specs conform to this
requirement.

Change-Id: I7b0c888a4dfd83306a470a4c9b0f9e44fe2e7818
Reviewed-on: https://go-review.googlesource.com/c/go/+/646016
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>